for those unsure of this, vodacom has been running a promotion in which you get a second amount of your monthly cap to surf between 12am and 5am. This doesn't carry over each month (as I found out last month 0_0), but is pretty useful.
so, for example, if you had 1GB of cap for this month, you'd also have an additional 1GB for the night owl times. So it effectively doubles your cap ^^
This is only available to vodacom contract clients.